Packages with metal line crack prevention design

ABSTRACT

A package includes a corner, a device die, a plurality of redistribution lines underlying the device die, and a plurality of metal pads electrically coupled to the plurality of redistribution lines. The plurality of metal pads includes a corner metal pad closest to the corner, wherein the corner metal pad is a center-facing pad having a bird-beak direction substantially pointing to a center of the package. The plurality of metal pads further includes a metal pad farther away from the corner than the corner metal pad, wherein the metal pad is a non-center-facing pad having a bird-beak direction pointing away from the center of the package.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/244,111, entitled “Packages with Metal Line Crack Prevention Design,”filed on Apr. 3, 2014, which application is incorporated herein byreference.

BACKGROUND

In the packaging of integrated circuits, there are various types ofpackaging methods and structures. For example, in a conventionalPackage-on-Package (POP) process, a top package is bonded to a bottompackage. The top package and the bottom package may also have devicedies packaged therein. By adopting the PoP process, the integrationlevel of the packages is increased.

In an existing PoP process, the bottom package is formed first, whichincludes a device die bonded to a package substrate. A molding compoundis molded on the package substrate, wherein the device die is molded inthe molding compound. The package substrate further includes solderballs formed thereon, wherein the solder balls and the device die are ona same side of the package substrate. The solder balls are used forconnecting the top package to the bottom package.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a cross-sectional view of a package in accordancewith some embodiments;

FIG. 2 illustrates a bottom view of a Redistribution layer (RDL) pad inaccordance with some embodiments, wherein the RDL pad incudes a main padregion and a bird-beak region connected to the main pad region;

FIGS. 3 and 4 illustrate bottom views of packages and RDL pads inaccordance with some embodiments, wherein center-facing pads andrandomly-pointed RDL pads are distributed depending on their distancesto neutral-stress points of the packages;

FIG. 5 illustrates a bottom view of some randomly-pointed RDL pads inaccordance with some embodiments;

FIG. 6 illustrates a bottom view of a package and RDL pads in accordancewith some embodiments, wherein center-facing pads are formed inrectangular corner regions of the package;

FIGS. 7 and 8 illustrate the bottom views of packages and RDL pads inaccordance with some embodiments, wherein the corner RDL pads adjacentto the corners of device dies have bird-beak directions pointing to thecenters of the respective device dies; and

FIGS. 9A through 9J illustrate bottom views of some exemplary corner RDLpads with respective to the corners of the device dies.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A package and the method of forming the package are provided inaccordance with various exemplary embodiments. The variations of theembodiments are discussed. Throughout the various views and illustrativeembodiments, like reference numbers are used to designate like elements.

FIG. 1 illustrates a cross-sectional view of package 20 in accordancewith some embodiments. Package 20 includes package 100 and package 200over and bonded to package 100. In some embodiments, package 100includes device dies 102, with the front sides of device dies 102 facingdown and bonded to Redistribution Layers (RDLs) 132/134/136. Throughoutthe description, the term “RDL” also refers to the redistribution linesin the redistribution layers. In alternative embodiments, package 100includes a single device die or more than two device dies. Device die102 may include semiconductor substrate 108, and integrated circuitdevices 104 (such as active devices, which include transistors, forexample) at the front surface (the surface facing down) of semiconductorsubstrate 108. Device die 102 may include a logic die such as a CentralProcessing Unit (CPU) die, a Graphic Processing Unit (GPU) die, a mobileapplication die, or the like.

Device dies 102 are molded in molding material 120, which surrounds eachof device dies 102. Molding material 120 may be a molding compound, amolding underfill, a resin, or the like. The bottom surface 120A ofmolding material 120 may be level with the bottom ends of device dies102. The top surface 120B of molding material 120 may be level with orhigher than back surface 108A of semiconductor substrate 108. In someembodiments, back surface 108A of semiconductor substrate 108 isoverlapped by die-attach film 110, which is a dielectric film adheringdevice die 102 to the overlying dielectric layer 118. Device die 102further includes metal pillars/pads 106 (which may include copperpillars, for example) in contact with, and bonded to, RDLs 132.

Package 100 may include bottom-side RDLs 132/134/136 underlying devicedies 102, and top-side RDLs 116 overlying device dies 102. Bottom-sideRDLs 132/134/136 are formed in dielectric layers 114, and top-side RDLs116 are formed in dielectric layers 118. RDLs 132/134/136 and 116 may beformed of copper, aluminum, nickel, titanium, alloys thereof, ormulti-layers thereof. In some embodiments, dielectric layers 114 and 118are formed of organic materials such as polymers, which may furtherinclude polybenzoxazole (PBO), benzocyclobutene (BCB), polyimide, or thelike. In alternative embodiments, dielectric layers 114 and 118 areformed of inorganic material such as silicon oxide, silicon nitride,silicon oxynitride, or the like.

Through-Vias 122 are formed to penetrate through molding material 120.In some embodiments, through-vias 122 have top surfaces level with thetop surface 120B of molding material 120, and bottom surfaces level withthe bottom surface 120A of molding material 120. Through-Vias 122electrically couple bottom-side RDLs 132/134/136 to top-side RDLs 116.Through-Vias 122 may also be in physical contact with bottom-side RDLs132 and top-side RDLs 116.

Electrical connectors 124, which are formed of a non-solder metallicmaterial(s), are formed at the bottom surface of package 100. In someembodiments, electrical connectors 124 include Under-Bump Metallurgies(UBMs), which are also metal pads. In alternative embodiments,electrical connectors 124 are metal pads, metal pillars, or the like.Metal pads 124 may comprise copper, aluminum, titanium, nickel,palladium, gold, or multi-layers thereof. In some embodiments, thebottom surfaces of metal pads 124 extend below the bottom surface of thebottom dielectric layer 114, as shown in FIG. 1. Solder regions 126 maybe attached to the bottom surfaces of metal pads 124.

In some embodiments, RDLs 132/134/136 include portions (including 132and 134) in more than one metal layers and vias 136 interconnecting theRDLs in different metal layers. For example, FIG. 1 illustrates RDLs132, which are closest to through-vias 122. The bottom surfaces ofthrough-vias 122 are in contact with some of RDLs 132. Furthermore,metal pillars 106 of device die 102 are also in contact with some ofRDLs 132. Electrical connectors 124 are electrically coupled to, and maybe in physical contact with, RDLs 134. Hence, RDLs 134 may be in themetal layer that is closest to electrical connectors 124. Vias 136 aredisposed between, and electrically interconnect, RDLs 132 and RDLs 134.

FIG. 2 illustrates a bottom view of one of RDLs 134. The illustrated RDL134 includes main pad region 138, metal trace 142, and bird-beak region140 connecting main pad region 138 to metal trace 142. In accordancewith some embodiments, main pad region 138 has a round bottom-viewshape. In alternative embodiments, main pad region 138 may have otherapplicable shapes including, and not limited to, rectangles, hexagons,octagons, and the like. bird-beak region 140 is the region that haswidths gradually and/or continuously transition from the width of mainpad region 138 to the width of metal trace 142. Metal trace 142 has oneend connected to one of vias 136, which leads to RDLs 132 (FIG. 1).

Arrow 144 is drawn to show the direction pointing from the center ofmain pad region 138 to the center (FIGS. 3, 4, and 6-8) of bird-beakregion 140. Direction 144 may also point from the center of main padregion 138 to the connecting point between bird-beak region 140 andmetal trace 142. Main pad region 138 and bird-beak region 140 incombination form an RDL pad 146 for supporting and connecting toelectrical connector 124 (FIG. 1). Throughout the description, direction144 is referred to the bird-beak direction of the respective RDL pad 146and the bird-beak direction of the respective RDL 134.

FIG. 3 illustrates an exemplary bottom view of bottom package 100,wherein RDL pads 146 (and some of RDL traces) are illustrated. RDLs 134include center-facing pads (sometimes referred to as center-facing metalpads hereinafter) 146A and randomly-pointed pads 146B. In FIGS. 3through 9J, circles are used to schematically represent randomly-pointedRDL pads 146B. Center-facing pads 146A, however, are illustrated withmore details to indicate their bird-beak directions. For example, themain pad region, the bird-beak region, and the respective bird-beakdirection of center-facing pads 146A are schematically illustrated.

FIG. 5 illustrates the bottom views (or top views) of exemplaryrandomly-pointed RDL pads 146B with more details, wherein therandomly-pointed RDL pads 146B are shown in FIGS. 3, 4, and 6 through9J. FIG. 5 schematically illustrate an inner region 153 (also refer toFIGS. 3 and 4) of package 100 and the randomly-pointed RDL pads 146Btherein. As shown in FIG. 5, randomly-pointed RDL pads 146B may have thesimilar shapes as center-facing pads 146A. For example, randomly-pointedRDL pads 146B may also include main pad regions, and bird-beak regionsconnected to the respective main pad regions. There are also metaltraces connected to the bird-beak regions, with the metal traces furtherconnected to vias 136.

As shown in FIG. 5, the bird-beak directions of randomly-pointed RDLpads 146B are randomly disposed, and can be in any directions.Therefore, each of the bird-beak directions of randomly-pointed pads146B may be in any direction, including center-facing andnon-center-facing. For example, the bird-beak directions ofrandomly-pointed RDL pads 146B do not necessarily point to the center ofpackage 100 (FIGS. 1 and 3), and do not necessarily point to the centerof any device die in package 100. Furthermore, neighboringrandomly-pointed RDL pads 146B may have different bird-beak directions.

Referring back to FIG. 3, bottom package 100 includes four corners 148.The corner RDL pads 146, which are closer to the respective corners 148than all other metal pads, are center-facing pads 146A, which have theirbird-beak directions pointing to (or substantially pointing to) center150 of package 100. Other RDL pads 146 that are farther away from therespective corners 148 than the corner RDL pads 146A arerandomly-pointed RDL pads 146B. In some embodiments, there may be morethan one center-facing pad 146A at each corner 148. For example, FIG. 4illustrates three center-facing pads 146A at each corner 148.

FIG. 4 also illustrates the bottom view of bottom package 100 and RDLpads 146 in accordance with alternative embodiments. In the bottom view,bottom package 100 has neutral-stress point 150, which is the point thatis substantially free from stresses from all lateral directions that areparallel to the bottom surface of package 100. At neutral-stress point150, the lateral stresses from opposite directions are cancelled out. Insome embodiments, neutral-stress point 150 is at or close to the center(also marked as 150) of bottom package 100 (in the bottom view). Thedistance of each of RDL pads 146 to neutral-stress point 150 is referredto as a Distance to Neutral Point (DNP), wherein the distance to the RDLpads 146 may be measured from a point of the RDL pad 146 that isfarthest to neutral-stress point 150. For example, DNPs DNP1 and DNP2are illustrated as examples in FIG. 4.

Referring to FIGS. 3 and 4, circle 152 is drawn with the neutral-stresspoint 150 as the center, wherein circle 152 has radius r. In accordancewith the embodiments of the present disclosure, all RDL pads 146 withthe DNPs equal to or smaller than radius r may be designed asrandomly-pointed RDL pads 146B, and all RDL pads 146 with DNPs greaterthan radius r are designed to be center-facing pads 146A. Some or allRDL pads 146 with the DNPs equal to or smaller than radius r may also bedesigned as center-facing pads 146A. As illustrated in FIG. 3, if radiusr is large, then the center-facing pads 146A may only include a singlecorner RDL pad 146 at each corner 148. In FIG. 4, radius r is reduced,and more corner RDL pads 146 are designed as center-facing pads 146A,while the RDL pads 146 with the DNPs equal to or smaller than radius rare randomly pointed, and do not necessarily point to the center 150.The optimum radius r of circle 152 may be determined by simulation orexperiment (by forming physical chips), so that the reliability of theRDLs 146 inside the circle 152 meets design specification.

In FIG. 4, the center-facing pads 146A at the same corner may have theirbird-bird directions parallel to each other. This means that thebird-beak directions of some of center-facing pads 146A (marked as146A′) are actually slightly offset from the center 150. In alternativeembodiments, all center-facing pads 146A at the same corner 148 may havebird-beak directions pointing right at center 150 of package 100, whichmeans that their bird-beak directions are substantially, but notexactly, parallel to each other.

FIG. 6 illustrates the design of RDL pads 146 in accordance with yetalternative embodiments. In these embodiments, four corner regions 154of bottom package 100 are defined, each extending from one of corners148 inwardly. The four corner regions 154 may have rectangular shapes,and may have sizes the same as each other. The RDL pads 146 insidecorner regions 154 are designed as center-facing pads 146A. The RDLsoutside corner regions 154 may be designed as randomly-pointed RDL pads146B, or may be designed as center-facing pads 146A.

In some embodiments as in FIG. 6, circle 152 is also drawn according tosimulation or experiment results. The radius of circle 152 may be small,and hence some of RDL pads 146 that are outside of corner regions 154are also outside of circle 152. Accordingly, as shown in FIG. 6, some ofRDL pads 146 (marked as 146″) that are outside of the circle 152 arealso center-facing pads 146A, while the RDL pads 146 that are outside ofcorner regions 154, but inside circle 152, are randomly-pointed RDL pads146B.

FIGS. 7 and 8 illustrate the bottom views of package 100 in theembodiments in which the RDL pads 146 adjacent to the corners of devicedie(s) 102 (FIG. 1) are also designed as center-facing pads 146A. Theseembodiments may be combined with the embodiments in FIGS. 3, 4, and 6,so that the center-facing pads 146A as shown in FIGS. 3, 4, and 6 mayalso be designed as center-facing pads 146A, in additional to the RDLpads 146A adjacent to the corners of device die(s) 102.

Referring to FIG. 7, device die 102 is illustrated. Device die 102includes corners 102A. At each of corners 102A, device die 102 overlaps(Refer to FIG. 1) at least a portion of one RDL pad 146A. Theneighboring RDL pads 146 adjacent to corners 102A are designed ascenter-facing pads 146A, wherein the neighboring RDL pads 146 arereferred to as corner RDL pads 146 hereinafter. The corner RDL pads 146of device die 102, instead of having bird-beak directions pointing tothe center 150 of package 100, have bird-beak directions pointing tocenter 156 of device die 102. The corner RDL pads 146, which arecenter-facing pads, may be fully surrounded by randomly-pointed RDLpads. Similarly, the corner RDL pads 146A close to the same corner ofdevice die 102 may have their bird-beak directions parallel to eachother, although the bird-beak directions may also point exactly tocenter 156, hence are substantially, but not exactly, parallel to eachother. Hence, throughout the description, when an RDL pad 146 isreferred to as “center-facing,” the bird-beak direction of the RDL pad146 may point to the center of the respective package, or the center ofa device die, depending on where the RDL pad is located.

FIG. 8 illustrates the bottom view of package 100 in accordance withalternative embodiments. These embodiments are similar to theembodiments in FIG. 7, except that there are two device dies 102disposed in bottom package 100. The corner RDL pads 146 that areadjacent to the corners of each of device dies 102 are designed ascenter-facing pads 146A. For each of the device dies 102, the respectivecorner RLD pads 146A have their bird-beak directions pointing to thecenter 156 of the respective device die 102.

FIGS. 9A through 9J are the exemplary embodiments for defining what arethe corner RDL pads of the device dies. Throughout FIG. 9A through 9J,nine RDL pads 146 are illustrated, and are marked with sequence numbersrange from 0 to 8, with the one with the sequence number 0 (referred toas the 0^(th) RDL pad 146 hereinafter) being the central one of the nineRDL pads 146. Furthermore, throughout FIGS. 9A through 9J, dx representsthe X-direction distance from the center of the 0^(th) RDL pad 146 tothe vertical edge 102B1 of device die 102, and dy represents theY-direction distance from the center of the 0^(th) RLD pad 146 to thehorizontal edge 102B2. Pitch P1 represents the pitches of neighboringRDL pads 146, which are, for example, the distances between the centersof the main pad region 138 (FIG. 2) of neighboring RDL pads 146.Furthermore, in subsequently recited equations, the value “a” representsthe diameter of RDL pads 146, as shown in FIG. 2. In each of the FIGS.9A through 9J, the RDL pads 146 in rectangular region 158 (referred toas corner region hereinafter) are defined as corner RDLs, and aredesigned as center-facing pads. Hence, the center-facing pads includethe 0^(th), the 1st, the 2nd, and the 4^(th) RDL pads 146. The 1st, the2nd, and the 4^(th) RDL pads 146 are the pads that are not overlapped bydevice die 102, and are closest to corner 102A. The remaining RDL pads146 may be randomly-pointed RDL pads, which may include the 3^(rd), the5^(th), the 6^(th), the 7^(th), and the 8^(th) RDL pads 146.

FIGS. 9A, 9B, and 9C illustrate the embodiments wherein the 0^(th) RDLpad 146 is fully overlapped by device die 102, and rest of the cornerRDL pads 146 are not overlapped by device die 102. For example, if oneof RDL pads 146 satisfies both of the following two relationships:a/2≤dx≤(P1−a/2)  [Eq. 1]a/2≤dy≤(P1−a/2)  [Eq. 2]the respective RDL pad 146 is the 0^(th) RDL, and the respective cornerregion 158 and the RDL pads in the corner region 158 can be identified,as illustrated. In FIG. 9A, the 0^(th) RDL pad 146 does not have anypoint overlapped by edges 102B1 and 102B2. In FIG. 9B, the 0^(th) RDLpad 146 has a point aligned to edge 102B1, and the 0^(th) RDL pad 146and device die 102 have no overlap. In FIG. 9C, dx is equal to (P1−a/2),which means that the 4^(th) RDL pad 146 has a point aligned to edge102B1, and the 4^(th) RDL pad 146 and device die 102 have no overlap.

FIGS. 9D, 9E, and 9F illustrate the embodiments wherein the 0^(th) RDLpad 146 are partially overlapped by the respective device die 102.Furthermore, the corner 102A of device die 102 also overlaps the 0^(th)RDL pad 146. For example, if one of RDL pads 146 satisfies both of thefollowing two relationships:dx<a/2  [Eq. 3]dy<a/2  [Eq. 4]the respective RDL pads 146 is the 0^(th) RDL, and the respective cornerregion 158 and the RDL pads in the corner region 158 can be identified.In FIGS. 9D and 9E, the centers of the 0^(th) RDL pad 146 are notoverlapped by the respective device dies 102. In FIG. 9F, the center ofthe 0^(th) RDL pad 146 is overlapped by device die 102.

FIGS. 9G through 9J illustrate the embodiments wherein the 0^(th) RDLpad 146 is partially overlapped by device die 102. Furthermore, edge102B2 overlaps the 0th RDL pad 146, while the corner 102A of device die102 does not overlap the 0^(th) RDL pad 146. For example, if one of RDLpads 146 satisfies both of the following two relationships:a/2≤dx≤(P1−a/2)  [Eq. 5]dy<a/2  [Eq. 6]the respective RDL pads 146 is the 0^(th) RDL pad, and the respectivecorner region 158 and the RDL pads in the corner region 158 can beidentified. In FIGS. 9G, 9H, and 9I, the centers of the respective0^(th) RDL pads 146 are overlapped by the respective device dies 102.Furthermore, FIGS. 9G, 9H, and 9I illustrate the embodiments in which dxis equal to, smaller than, and greater than, (P1)/2. In FIG. 9J, thecenter of the 0^(th) pad RDL pad 146 is not overlapped by device die102.

The embodiments of the present disclosure have several advantageousfeatures. The RDL pads that are close to the corners of package 100 anddevice die 102 suffer from high stresses, and hence the RDL traces ofthese RDL pads are more likely to be broken by the stresses. Experimentresults and simulation results indicate that the center-facing pads aremore reliable, and the stresses suffered by the traces connected to thecenter-facing pads are lower than the stresses suffered by therandomly-pointed RDL pads. Accordingly, by designing the RDL pads thatsuffer from higher stresses as center-facing, the reliability of therespective package is improved. On the other hand, the RDL pads sufferfrom low stresses may have their bird-beak directions pointing randomlyto improve the flexibility in RDL routing.

In accordance with some embodiments of the present disclosure, a packageincludes a corner, a device die, a plurality of redistribution linesunderlying the device die, and a plurality of metal pads electricallycoupled to the plurality of redistribution lines. The plurality of metalpads includes a corner metal pad, wherein the corner metal pad is acenter-facing pad having a bird-beak direction substantially pointing toa center of the package. The plurality of metal pads further includes ametal pad farther away from the corner than the corner metal pad,wherein the metal pad is a non-center-facing pad having a bird-beakdirection pointing away from the center of the package.

In accordance with alternative embodiments of the present disclosure, apackage includes at least one first dielectric layer, a first pluralityof redistribution lines in the at least one first dielectric layer, adevice die over and electrically coupled to the first plurality ofredistribution lines, a molding material molding the device die therein,a through-via penetrating through the molding material, and at least onesecond dielectric layer over the device die. A second plurality ofredistribution lines is in the at least one second dielectric layer. Thesecond plurality of redistribution lines is electrically coupled to thefirst plurality of redistribution lines through the through-via. Aplurality of metal pads is underlying the device die and electricallycoupled to the second plurality of redistribution lines. The pluralityof metal pads includes a first center-facing metal pad and anon-center-facing metal pad.

In accordance with yet alternative embodiments of the presentdisclosure, a package includes a plurality of dielectric layers, aplurality of redistribution lines in the plurality of dielectric layers,a device die over and electrically coupled to the plurality ofredistribution lines, and a plurality of metal pads underlying andelectrically coupled to the plurality of redistribution lines. Theplurality of metal pads includes a corner metal pad, wherein the cornermetal pad has a first bird-beak direction pointing to a first center ofa package that includes the plurality of metal pads and the device die.The plurality of metal pads further includes an inner metal pad adjacentto a corner of the device die, wherein the inner electrical has a secondbird-beak direction pointing to a second center of the device die. Theplurality of metal pads also includes a plurality of non-center-facingmetal pads surrounding the inner metal pad.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A package comprising: a first package comprising:a device die comprising a topmost surface at a first vertical level anda bottommost surface at a second vertical level; a through-via extendingfrom the first vertical level to the second vertical level; anencapsulant encapsulating the device die and the through-via therein; aplurality of redistribution lines over the device die; and a pluralityof metal pads over and electrically coupling to the plurality ofredistribution lines, wherein the plurality of metal pads comprise: anarray comprising a plurality of rows and a plurality of columns of metalpads, wherein each of metal pads in the array comprises a roundedportion and a bird-beak portion joining the rounded portion, wherein thearray comprises a corner metal pad closest to a corner of the firstpackage, wherein the corner metal pad is electrically connected to thethrough-via, and wherein the corner metal pad is a center-facing padwith a bird-beak pointing direction pointing from a center of therounded portion of the corner metal pad to a tip of the bird-beakportion of the corner metal pad being substantially pointing to a centerof the first package, and wherein bird-beak pointing directions of allmetal pads in the array are parallel to each other; and anunder-metal-metallurgy (UBM) contacting the rounded portion of thecorner metal pad.
 2. The package of claim 1, wherein a first end of thebird-beak portion of the corner metal pad is joined to the roundedportion of the corner metal pad, and a second end of the bird-beakportion is the tip, and is joined to a metal trace, and wherein themetal trace has a width smaller than a diameter of the rounded portion.3. The package of claim 1, wherein the through-via comprises a straightedge extending from the first vertical level to the second verticallevel.
 4. The package of claim 1, wherein at each of corners of thefirst package, there is one of the plurality of metal pads that has arespective bird-beak pointing direction substantially pointing to thecenter of the first package.
 5. The package of claim 1, wherein theplurality of metal pads further comprise an inner metal pad farther awayfrom the corner than the corner metal pad, wherein the inner metal padcomprises an additional bird-beak, and a bird-beak pointing direction ofthe inner metal pad points away from the center of the first package. 6.The package of claim 1 further comprising an additional metal padfarther away from the corner than the corner metal pad, wherein theadditional metal pad has a circular top view.
 7. The package of claim 1,wherein the device die further comprises a metal pillar, wherein a topsurface of the metal pillar being a part of the topmost surface of thedevice die, and wherein sidewalls of the metal pillar are in contactwith the encapsulant to form vertical interfaces.
 8. The package ofclaim 1, wherein the center of the first package is a neutral-stresspoint, wherein all metal pads that are in the plurality of metal padsand have Distances-to-Neutral-Point (DNPs) greater than a radius haverespective bird-beak pointing directions facing the neutral-stresspoint.
 9. The package of claim 8, wherein all metal pads that are in theplurality of metal pads and have DNPs smaller than a radius havecircular top view shapes.
 10. The package of claim 8, wherein at leastsome metal pads in the plurality of metal pads and having DNPs equal toor smaller than the radius have bird-beaks pointing to directions otherthan the center of the package.
 11. The package of claim 1, wherein theplurality of metal pads are on a front side of the device die.
 12. Thepackage of claim 1 further comprising a second package bonded to thefirst package, wherein the UBM is between the device die and the secondpackage.
 13. A package comprising: a device die comprising a center; anencapsulating material encapsulating the device die therein; and aplurality of metal pads in a same layer underlying the device die, withsome of the plurality of metal pads electrically coupling to the devicedie, wherein the plurality of metal pads comprise: a corner pad, whereinthe corner pad is overlapped by the device die, and is closer to acorner of the device die than all other metal pads overlapped by thedevice die, wherein in a plane view of the package, a bird-beakdirection of the corner pad points to the center of the device die. 14.The package of claim 13 further comprising a center-facing metal pad anda non-center-facing metal pad, wherein the center-facing metal padcomprises a circular portion and a bird-beak portion joining thecircular portion, and a bird-beak pointing direction that points from acenter of the circular portion to the bird-beak portion points to acenter of the package, wherein the center-facing metal pad is a cornermetal pad closest to a corner of the package.
 15. The package of claim13, wherein the plurality of metal pads further comprise multiple metalpads closer to the corner of the device die than other metal pads,wherein the multiple metal pads have bird-beak directions pointing tothe center of the device die.
 16. The package of claim 13, wherein theplurality of metal pads comprise a plurality of corner metal pads at asame corner of the package, and the plurality of corner metal pads havebird-beak pointing directions parallel to each other.
 17. A packagecomprising: a first device die; and a plurality of metal pads underlyingthe first device die, wherein the plurality of metal pads comprise: afirst array of metal pads comprising a first plurality of rows and afirst plurality of columns of metal pads, wherein each of the firstarray of metal pads comprises a first rounded portion and a firstbird-beak portion joining the first rounded portion, with bird-beaks ofthe first array of metal pads all pointing to a same direction, whereinthe first array of metal pads comprises a first corner metal pad at afirst corner of the package; and a second corner metal pad at a secondcorner of the package, wherein the second corner metal pad comprises asecond rounded portion and a second bird-beak portion joining the secondrounded portion, wherein the first bird-beak portion and the secondbird-beak portion point to each other.
 18. The package of claim 17,wherein in a top view of the package, both the first bird-beak portionand the second bird-beak portion point to a center of the package. 19.The package of claim 17 further comprising a second array of metal padscomprising a second plurality of rows and a second plurality of columnsof metal pads, with bird-beaks of the second array of metal pads allpointing to a same direction, wherein the second array of metal padscomprises the second corner metal pad.
 20. The package of claim 17,wherein in a top view of the package, the first corner and the secondcorner are on a first straight line, and a center of the first cornermetal pad and a center of the second corner metal pad are on a secondstraight line, and wherein the first straight line offsets from thesecond straight line.
 21. The package of claim 1, wherein the pluralityof metal pads further comprise an additional metal pad overlapping thedevice die, wherein the additional metal pad is closer to a corner ofthe device die than all other metal pads overlapping the device die, andwherein in a plane view of the package, a bird-beak direction of theadditional metal pad points to a center of the device die.